1. Field of the Invention
The present invention relates to a magnetic tape cassette. More particularly, it relates to an improvement of a screw holder receiving a screw for connecting an upper half casing and a lower half casing after the assembling of said two half casings.
2. Description of the Prior Art
The magnetic tape cassette usually comprises the upper half casing, a lower half casing to be assembled with the upper half casing and a magnetic tape wound on a hub which is held in the assembly of the two half casings. Therefore, screw holders, also known as screw receivers, are formed at the four corner of each half casing for connecting the assembled upper and lower half casings with screws.
FIG. 1 is a cross sectional view of one corner portion of the lower half casing (1) of the conventional magnetic tape cassette. As shown in the figure, a screw holder (2) is formed in the corner portion. The screw holder (2) is integrally connected to the front wall (4) and the side wall (6) of the lower half casing. The reference numeral (8) designates a guide pole, (10) designates a guide roller and (12) designates an M shaped rib.
The lower half casing (1) has a fitting surface (3) on which the upper half casing (20) is placed, as shown in FIG. 2. The screw holder (2) of the lower half casing is provided with an annular boss (5) projecting upward from the fitting surface, while the upper half casing (20) is provided with a caved portion (22) for receiving the annular boss (5) at the corresponding position; thus the positions of the lower half casing (1) and the upper half casing (20) are determined. A threaded hole is formed in the annular boss (5) and the screw holder (2). The thickness of wall of the annular boss (5) is smaller than that of the screw holder (2) so that stop shoulders (2'),(2") are provided on the upper surface of the screw holder (2), namely the fitting surface (3) to support the upper half casing (20). Accordingly, when a tapping screw (30) is threaded through a counter-sunk hole (24) of the upper half casing (20) into the threaded hole (14) of the screw holder (2), the upper half casing (20) is brought into contact with the fitting surface (3) and the stop shoulder (2') of the lower half casing (1) to provide an integral connection between them. However, it is necessary to provide a substantial thickness of the wall of the screw holder (2) to provide the stop shoulders (2'), (2") on the screw holder (2) of the lower half casing (1). The provision of the stop shoulders results in a large recess on the reverse side surface (7) of the screw holder as the reverse surface of the tape cassette during the molding operation; thus the accuracy of size such as parallel shape, flatness of the magnetic tape cassette is greatly reduced and the appearance is spoilt.
In order to overcome the disadvantage of occurrence of "recess" in the screw holder, an attempt in which the stop shoulder (2") placed inside the screw holder (2) (FIG. 2) is removed and the thickness of wall of the screw holder (2) is reduced, that is, it has a thickness the same as that of the annular boss (5) as shown in FIGS. 3 and 4, has been proposed. This attempt substantially reduces the occurrence of the "recess" while the area of the stop shoulder of the screw holder (2) is reduced by as much as about half. The upper half casing is, therefore, easily deformed thus not being completely parallel.